The Non‐Ancillary Nature of Trimethylsilylamide Substituents in Boranes and Borinium Cations

Abstract The known boranes (R(Me3Si)N)2BF (R=Me3Si 1 , tBu 2, C6F5 3, o‐tol 4, Mes 5, Dipp 6) and borinium salts (R(Me3Si)N)2B][B(C6F5)4] (R=Me3Si 7, tBu 8) are prepared and fully characterized. Compound 7 is shown to react with phosphines to generate [R3PSiMe3]+ and [R3PH]+ (R=Me, tBu). Efforts to generate related borinium cations via fluoride abstraction from (R(Me3Si)N)2BF (R=C6F5 3, o‐tol 4, Mes 5) gave complex mixtures suggesting multiple reaction pathways. However for R=Dipp 6, the species [(μ‐F)(SiMe2N(Dipp))2BMe][B(C6F5)4] was isolated as the major product, indicating methyl abstraction from silicon and F/Me exchange on boron. These observations together with state‐of‐the‐art DFT mechanistic studies reveal that the trimethylsilyl‐substituents do not behave as ancillary subsitutents but rather act as sources of proton, SiMe3 and methyl groups.

A series of silyl-aryl amides were reacted with (OEt 2 )BF 3 , affording the compounds (R(Me 3 Si)N) 2 BF (R=Me 3 Si 1, tBu 2, C 6 F 5 3, o-tol 4, Mes 5, Dipp 6) (Scheme 1) via a minor modification of the known literature protocol. [18] These compounds exhibited 11 B NMR signals at ca. 23-24 ppm, characteristic of threecoordinated boron centers and showed 19 F signals attributable to the BF fragment in the range from À 85.5 to À 107.7 ppm. Additionally, in the case of 3, resonances at À 147.2, À 160.8 À 165.2 ppm arose from the C 6 F 5 rings. Compounds 3 and 6 were characterized by X-ray crystallography ( Figure 2) exhibiting the expected three-coordinated geometry at boron. The NÀ BÀ N angles in 3 and 6 were found to be 128.9(7)°and 135.5(2)°, respectively. The wider angle in 6 reflects the steric demands of the aryl substituents. The corresponding BÀ N distances average 1.418(3) Å and 1.424(5) Å, while the BÀ F distances are 1.362(9) Å and 1.377(3) Å, respectively. The significantly shorter BÀ F bond in 3 is consistent with the presence of the electron withdrawing C 6 F 5 rings. It is interesting to note the π-stacking of the aryl rings as this dictates that the SiMe 3 groups flank the BÀ F units.
The borinium cation salt [((Me 3 Si) 2 N) 2 B][B(C 6 F 5 ) 4 ] 7, was prepared using the reaction of [Et 3 Si(tol)][B(C 6 F 5 ) 4 ] with 1 in odifluorobenzene ultimately affording a white product in 43 % yield. The 1 H and 13 C{ 1 H} NMR data for the cation were consistent with previously reported data, while the 11 B and 19 F NMR spectra were consistent with the formulation of 7. In addition, a crystallographic study confirmed the connectivity of this salt (Figure 3). The central boron of the cation is a linear and two-coordinated with BÀ N bond distances of 1.330(4) and 1.328(4) Å. The compound 7 is directly analogous to the [BBr 4 ] À salt originally reported by . [19] The DFTcomputed Wiberg bond index of 1.52 for this cation of 7 is consistent with some degree of BÀ N π-bonding.
In a similar fashion, the analogous reaction of [tBu(Me 3 Si)N] 2 BF afforded the species [(tBu(Me 3 Si)N) 2 B][B(C 6 F 5 ) 4 ] 8. This product exhibited the expected 1 H and 13 C NMR spectra as well as the 11 B resonances at 34.6 and À 16.6 ppm, corresponding to the anion and cation, respectively. Crystallographic data confirmed the connectivity (Figure 3). One of the amido-groups is disordered via interchange of the tBu and Me 3 Si groups. Nonetheless, the other amide group is not disordered, revealing NÀ C and NÀ Si distances of 1.538(5) Å and 1.820(3) Å with two BÀ N distances of 1.324(6) Å and 1.346(6) Å, respectively.
To gain mechanistic insights, extensive dispersion-corrected DFT calculations were performed at the PW6B95-D3 + COSMO-RS//TPSSÀ D3/def2-TZVP + COSMO level in CHCl 3 solution and the final free energies were reported in kcal/mol, at 298 K and 1 mol/L concentration [20] (Figure 4). The separated ions of [Et 3 Si-(tol)] + and [B(C 6 F 5 ) 4 ] À in solution provide a low barrier to fluoride abstraction from 1 of 11.9 kcal/mol (via transition state    TS1 + ) providing the borinium cation of 7, toluene and Et 3 SiF in an overall exergonic process (À 23.5 kcal/mol). Interestingly, methyl abstraction from 1 with [Et 3 Si(tol)] + (via TS2 + ) was computed to be nearly neutral in free energy over a low barrier of 12.6 kcal/mol affording the boryliminium cation A + , from which an intramolecular fluoride shift (via TS3 + ) is almost barrierless and À 23.0 kcal/mol exergonic to form another borinium cation 7F + . While not directly observed experimentally, this alternative reaction pathway could account for the moderate yield of 7.
DFT calculations were also used to probe the stabilizing effect of hyperconjugation between the silyl groups and the cationic boron center in 7 and 8. The free energies of the isodesmic reactions with NH 3 to give [(H 2 N) 2 B] + and HN-(SiMe 3 )(R) (R=SiMe 3 and tBu) were found to be 15.8 (4 SiMe 3 groups) and 12.8 (2 SiMe 3 groups) kcal/mol respectively. These results are consistent with about 3.9 and 2.4 kcal/mol smaller than a NÀ H bond for each NSiMe 3 and NtBu group, respectively. Initial monitoring of the reaction of 7 with PMe 3 showed a broad 11 B peak at ca. 34.3 ppm while the 31 P resonance was observed as a broad signal at À 49.2 ppm. These data suggest the coordination of phosphine to boron, generating the proposed species [((Me 3 Si)N) 2 BPMe 3 ][B(C 6 F 5 ) 4 ]. This view was supported by DFT computed 11 B and 31 P signals at 33.5 and À 43.0 ppm, respectively. However, this species was not isolable, as it proved transient. Nonetheless, cooling to À 30°C afforded an insoluble product 9 which was isolated as colourless crystals albeit in low yield. A crystallographic study confirmed the formulation of 9 as [Me 3 PSiMe 3 ][B(C 6 F 5 ) 4 ]. The metric parameters of the cation [Me 3 PSiMe 3 ] + are identical to those reported for its triflate salt, [21] although it is noteworthy that the cation of 9 adopts an conformation in which the methyl groups on P and Si are staggered (Scheme 2). Also of interest, the 31 P shift of 9 is ca. 26 ppm downfield of the triflate salt, [21] consistent with the coordination of the triflate in solution. The formation of 9 results from the net loss of [SiMe 3 ] + from 1 generating the byproduct [(Me 3 Si) 2 NB(NSiMe 3 )] 2 10 a species previously prepared by dehydrohalogenation of (Me 3 Si) 2 NBCl(NHSiMe 3 ) with BuLi. [22] The observed 11 B resonance at 27.1 ppm is consistent with this formulation, and corroborated by a recent report, [23] although exhaustive efforts to isolate this by-product were unsuccessful.
DFT calculations also revealed that the coordination of PMe 3 to the borenium cation 7 + is 6.8 kcal/mol endergonic, to give the kinetically accessible but thermodynamically unstable borenium cation [((Me 3 Si)N) 2 BPMe 3 ] + , B + ( Figure 5), analogous to that proposed on the basis of experimental data. Alternatively, PMe 3 may abstract a silylium cation [SiMe 3 ] + from 7 + , which is 4.6 kcal/mol endergonic over a low barrier of 8.9 kcal/ mol (via TS4 + ) to form the observed cation 9 + and neutral (Me 3 Si) 2 NB=NSiMe 3 . Subsequent dimerization of the latter species is energetically favoured affording the product 10, making the overall reaction À 12.7 kcal/mol exergonic. In contrast, borinium-assisted methyl deprotonation of the cation of 7 with PMe 3 is 1.3 kcal/mol endergonic over a sizable barrier of 24.0 kcal/mol (via TS5 + ), which is thus both kinetically and thermodynamically disfavored. The use of bulky tBu 3 P evidently increases the silylium abstraction barrier to 18.6 kcal/mol, while the methyl deprotonation channel becomes À 5.0 kcal/mol exergonic though kinetically still less competitive. The corresponding reaction of 7 with tBu 3 P gave a complex mixture of products. The observation of 31 P resonances at 61.4 and 30.4 ppm (DFT: 53.2 and 21.0 ppm) were consistent with the formation of [tBu 3 PH] + and the previously reported cation [tBu 3 PSiMe 3 ] + , [24] respectively. Crystals of [tBu 3 PSiMe 3 ][B(C 6 F 5 ) 4 ] 11 were isolated from the reaction albeit in low yield and the connectivity was confirmed by preliminary crystallographic data although disorder of the tert-butyl groups precluded publication. The formation of these two phosphonium cations suggests that the borinium cation acts as a source of both proton and SiMe 3 suggesting the formation of [(Me 3 Si) 2 NB(N-(SiMe 3 )SiMe 2 CH 2 ] and 10, respectively. Again despite exhaustive efforts, these species could not be isolated, however these species were proposed based on computational data ( Figure 5).
The formation of 12 is consistent with silylium abstraction of a methyl group from Si, and a fluoride for methyl from boron to silicon. In a very recent report by Chiu [25] and coworkers, the transient generation of [MesBN(SiMe 3 ) 2 ] + was observed to prompt methyl migration from Si to B. In that case, it gave the four-membered-ring species (Me 3 Si)N(SMe 2 )(μ-Mes)BMe. To probe the present reaction further, DFT computations were performed. These revealed that selective fluoride-assisted methyl-abstraction from aryl-substituted 6 by [Et 3 Si(tol)] + is À 8.9 kcal/mol exergonic over a low barrier of 8.3 kcal/mol (via TS6 + ) affording the intermediate cation D + with loss of Et 3 SiMe and toluene (Figure 7). Further a Si-to-B methyl-shift (via TS7 + ) and even faster ring-closing through BÀ N bond rotation of E + (via TS8 + ) is À 12.0 kcal/mol exergonic over a higher barrier of 13.3 kcal/mol eventually affording the SiÀ FÀ Si bridged cation 12 + . In contrast, direct fluoride abstraction from 6 with [Et 3 Si-(tol)] + is À 10.1 kcal/mol exergonic over a sizable barrier of 24.7 kcal/mol (via TS6a + , see Supporting Information) and is thus kinetically much less favorable, mainly due to enhanced steric hindrance and restrained BÀ N bond rotation of the bulky Dipp groups.
The above reactivity demonstrates that borinium cations are accessible via fluoride abstraction from 1 and 2. These species react with phosphines readily undergo competitive desilylation or silylmethyl deprotonation reactions. In contrast, efforts to generate related borinium cations from 3-5 gave complex mixtures, suggesting multiple reaction pathways. The isolation of the heterocyclic cation 12 from the corresponding reaction of 6 demonstrates methyl abstraction from silicon as an alternative reaction pathway. Generally, these observations demonstrate that the silylamides in borane and borinium precursors are not ancillary ligands acting as sources of SiMe 3 + , H + and Me groups. We are continuing to probe the reactivity of two-coordinated boron cations, their Lewis acidity and potential use in catalysis.

Crystallographic details
Deposition Numbers 2125379 (for 1) 2125380 (for 12) 2125381 (for 2) 2125382 (for 6) 2125383 (for 9) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.